This is a revision of a competing renewal application which was approved but not funded. In this revision we have made every effort to respond to the critiques of the prior review. Behavioral economics, the application of microeconomic principles to the study of behavior, has value for the field of drug dependence. The value stems from (1) its ability to integrate the effects of independent variables into a singular term, referred to as unit price, and (2) its ability to specify conditions under which choice of a drug vs. a non-drug reinforcer varies from being probable to improbable. The former suggests the parsimony of this approach; the latter suggests its utility in accounting for the selection of drug vs. other reinforcers. The purpose of this proposal is two-fold. The first purpose is to continue our basic research on unit price. We will examine whether several important operations that have yet to be incorporated into unit price (e.g., punishment, reinforcer delay, & probability of reinforcement) can be parsimoniously integrated. These experiments will demonstrate the generality and define the limitations of unit-price. The second purpose is to examine the utility of behavioral economics in the evaluation of medications for the treatment of drug dependence. We will examine the effect of agonist and antagonist treatment on the economic measure of elasticity (the responsiveness of drug consumption to unit price). This new measure, elasticity, may provide an index of whether a medication renders drug use more sensitive to cost factors in the environment. Further, we will examine whether the behavioral- economic determinants of drug choice can assist in assessing the effectiveness of medications under different environmental conditions and whether those determinants can be used to develop a laboratory analog of conditions that promote patient entry into treatment. These experiments will put what we have learned about behavioral economics into the service of assessing medications for the treatment of drug dependence. Moreover, the behavioral-economic determinants of drug choice will allow us to assess and characterize medications under conditions that may more closely approximate clinical conditions than do current assessment models. Finally, developing an assay to assess factors that determine entry into treatment may be useful for any form of treatment. Overall, the continued assessment of the basic factors related to the behavioral economics of drug self-administration and the application of those factors to address medication development provide an important opportunity to explore and utilize this new approach.